4 Answers
4

From practical experience we've found that beyond the usual 301 redirecting these following actions have resulted in a shorter (and on one occasion non-existent) Search Engine fluctuation:

Time the migration well away from your domain name expiry/renewal, so there is little ambiguity over it being a different site

Set all far-future HTTP expiry headers to the date/time that the website is to be changed, relative timestamps require a little fiddling to become absolute when they are close enough

Migrate slowly, backporting as much as you can so the site runs in parallel - Google has a learning process to "trust" your site - if all paths are suddenly 301's it sometimes backs off indexing only to come back a day or two later, during which time your ranking plumets

Have your new site structure up early (and in parallel) using sitemaps to usher search engines towards the correct URL paths - when it gets to the point that you're now using 301 messages to enforce the change, there'll be additional trust between the Search Engines and yourself

Remember that if you find that there will be duplicate URL paths, use the

Good advice. Also, in Google Webmaster Tools under "Site Configuration" -> "Change of Address" there is a tool to enter the new verified domain; not sure if this helps Google associate the results more quickly, but it doesn't hurt.
–
JasonBirchAug 31 '10 at 2:39

"Migrate slowly, backporting as much as you can so the site runs in parallel" - could you possibly explain this in a bit more detail?
–
AndyAug 31 '10 at 11:19

If you are changing the "theme" of the site, try to change the mark-up to match the new site (one element per week, etc) and then changeover the CSS at the last minute. If the JavaScript libraries are to be updated try to make them run on the existing site and transfer them over (you can also start to post-load any JavaScript libraries to be used on the new site a week before the change, so clients have them cached). Match up META/Title information. Basically changing everything at once often causes Search Engine rankings to tumble, make that change as simple to follow as possible.
–
MetalsharkAug 31 '10 at 11:33

If your URLs are changing be sure to do a 301 redirect so the search engines know that the old URL has moved to a new location and all incoming links for the old URL should be transferred to the new URL. A sample 301 redirect in .htaccess would look like this:

redirect 301 /old.html http://www.example.com/new.html

That's about all you can control when you redesign a web site. Because your HTML will be changing your rankings may change, too, as semantic markup very much affects your rankings. The best you can do is make sure you properly markup your content (use heading tags, etc) take advantage of every internal linking opportunity.

The biggest thing is to make sure that you know what all your current URLs are and then redirect them to the new URLs using a 301 redirect when you redesign. You may find that your rankings take a slight dip temporarily, but don't panic, that can sometimes happen as Google readjusts itself to your site. Wait it out a few weeks before doing anything drastic.

If you are able, you could also change your anchor href's to match the new URLs.

In your experience have you witnessed loss of google rankings for the majority or website redesigns?
I am looking to get my site redesigned and potentially a new cms but I'm totally put off by cases of sites going into oblivion after a redesign.